One of the things to think about is just how much damage would be done to joker’s body over time as a non-superpowered human. Joker’s getting kicked to pieces repeatedly. He’s not getting Bruce Wayne’s concierge medical care. He’d be a maraca of bone shards after the first few beatdowns. Now, he’s crazy, so maybe he doesn’t even notice the pain, but you have to expect he’d be going around with a garishly colored walker after the twentieth time being thrown through a window onto concrete.